Jay--I have some of that going on. I did a web search and more than one site talked about incomplete pollination due mostly to not enough visits from bees. My bee population does seem to be down this year so next year I will put some bee attracting flowers in with the vegetables early in the season to attract pollinators.

Incomplete pollination seems right, per the above article, though I thought it happened mostly to much younger cucs, which would just shrivel up and die. Mine are quite mature, it would seem, for this to happen. They are healthy and green, so I'm surprised. Anyway, it's likely still the problem.